1. Field of the Invention
The present invention relates to an optical pickup and optical device for use in a playback unit for an information recording medium such as an optical disc and more particularly to an optical pickup and optical device preferable for a compatible playback system for digital versatile disc (DVD) and compact disc-write once (CD-R).
2. Description of the Related Art
Currently, a DVD system has been proposed and marketed, and its prevalence has started, the DVD system having a higher density than an optical compact disc (CD) which has already prevailed as a consumer product. In DVD player as a playback unit of this system, compatible playback with CD is indispensable to avoid an overlapping of the devices and complexity of operation upon use. Likewise, the DVD player is also required to be compatible with the CD-R which can be played back by the CD player. A technology for playing back discs of various standards has been developed and further, simplification and reduction of cost for achieving it are problems to be solved.
For the aforementioned CD-R, a laser beam source having 780 nm band laser beam source different from 650 nm band for the DVD is required because reflectivity of an information recording medium has a high wave length dependency and therefore, an optical pickup containing a light source for two wave lengths (wavelengths) is necessary.
On the other hand, integration of optical system of the optical pickup has been demanded for the purpose of cost reduction and size reduction, and for this integration of the optical system, making a common optical axis of light from a light source having two wave lengths is effective. This is because by making common the optical axis, the optical path can be made single, and optical parts can be used in common and the like advantages exist.
As an optical system satisfying these requirements, such a structure in which diffraction gratings are disposed on the same optical axis and diffracted lights having each wave length (wavelength) are received by respective light receiving devices (photodetectors) can be considered.
However, it is very difficult to design such a diffraction grating as to change optical paths of lights having different wave lengths appropriately and to be provided with lens power (lens action). The reason is that because characteristics such as diffracting angle of the diffraction grating is determined depending on a mathematical relation between its cyclic structure and wave length of light, accordingly the diffracting angle changes largely with respect to a different wave length and further characteristics about optical path change and lens power (lens action) also change largely.
Thus, if lights having different wave lengths are emitted to the same diffraction grating, an optimum light receiving plane differs depending on the above-described characteristic change, so that sharing of the light receiving device (photodetector) substrate cannot be achieved, thereby obstructing a further integration of the optical system.